1. Field of the Invention
The present invention generally relates to a method for fabricating embedded thin film resistors of a printed circuit board, and more specifically to a method of fabricating one or more embedded resistors in a printed circuit board.
2. The Prior Arts
In general, besides using conventional discrete passive elements, a printed circuit board can also use a thick film or a thin film process to develop the embedded resistors required. In the thick film process, the resistors of the printed circuit board are made of carbon paste printed on the printed circuit board. Then the resistances of the resistors are fine-tuned by the laser trimming after drying. In the thin film process, on the other hand, a nickel-silicide-plated copper foil is used to replace the pure copper foil. The nickel-silicide-plated copper foil and the epoxy resin of the printed circuit board are pressed together during the fabricating process of the printed circuit board. The nickel-silicide-plated side of the copper foil faces toward the laminate of the printed circuit board and the non-plated side of the copper foil faces outward. Then, in a subsequent photolithography process, an acid etching solution is first used to etch both the copper and nickel-silicide layers, and then an alkaline etching solution is used to etch away the copper layer above the embedded resistors. One or more nickel-silicide blocks with the required dimensions are thereby formed. Laser is then used to trim each of the nickel-silicide blocks to achieve the precise resistance required.
In addition, U.S. company, Macdermid, developed a process to fabricate embedded resistors by adopting the nickel-phosphorus or palladium-phosphorus deposition technology conventionally used in industry. The process includes circuitry fabrication, activation, forming photosensitive etching resist, lithographic etching to expose resistor locations, immersing in an electroless dip solution, removing photosensitive etching resist, and fine-tuning resistance with laser trimming to accomplish the fabrication of embedded thin film resistors.